CO₂-driven ocean acidification weakens mussel shell defense capacity and induces global molecular compensatory responses

Oceanic uptake of atmospheric CO₂ is reducing seawater pH and shifting carbonate chemistry within, a process termed as ocean acidification (OA). Marine mussels are a family of ecologically and economically significant bivalves that are widely distributed along coastal areas worldwide. Studies have demonstrated that OA greatly disrupts mussels' physiological functions. However, the underlying molecular responses (e.g., whether there were any molecular compensation mechanisms) and the extent to which OA affects mussel shell defense capacity remain largely unknown. In this study, the thick shell mussels Mytilus coruscus were exposed to the ambient pH (8.1) or one of two lowered pH levels (7.8 and 7.4) for 40 days. The results suggest that future OA will damage shell structure and weaken shell strength and shell closure strength, ultimately reducing mussel shell defense capacity. In addition, future OA will also disrupt haemolymph pH and Ca²⁺ homeostasis, leading to extracellular acidosis and Ca²⁺ deficiency. Mantle transcriptome analyses indicate that mussels will adopt a series of molecular compensatory responses to mitigate these adverse effects; nevertheless, weakened shell defense capacity will increase mussels' susceptibility to predators, parasites and pathogens, and thereby reduce their fitness. Overall, the findings of this study have significant ecological and economic implications, and will enhance our understanding of the future of the mussel aquaculture industry and coastal ecosystems.